Potato slices can be used to make different types of potato products. Potato chips, for example, are made by frying potato slices in hot oil, and then applying a seasoning, such as salt. Potato slices can also be baked to make low fat potato chips and other snack products. Sometimes the products are frozen and then reheated by food service establishments and consumers for consumption. In addition, potato slices can be partially cooked, dehydrated, and then rehydrated for preparing casseroles, soups, salads and entrees. Dehydrating of potato slices permits storage of the slices for long periods of time, and rehydration on an "as needed" basis.
Often the preparation of fried, baked or dehydrated potato slices results in pillowing and blistering in the finished potato product. FIGS. 1A-1D illustrate a potato product 10, in the form of a fried potato chip, that has undergone pillowing and blistering. Pillowing refers to an uncontrolled separation of major portions of opposing surfaces of the potato product 10. FIG. 1C illustrates a pillow 12 in the potato product 10. Blistering is similar to pillowing except that the separation occurs from only one surface of the potato product 10. FIG. 1D illustrates a blister 14 in the potato product 10.
One explanation for pillowing and blistering is that during the cooking process starch in outer layers of the potato slice gelatinizes and loses moisture more rapidly than starch in the center of the potato slice. This forms a "skin" on the outer surface of the potato slice. In addition, steam can be trapped within the intercellular spaces of the potato slice. The steam forces the cells apart as the pectic substances between adjacent cell walls softens and solubilizes. However, the "skin" retards the escape of steam, causing the trapped steam to form a pillow or a blister.
Pillowing and blistering can adversely affect the appearance, texture and taste of potato products. For example, pillows and blisters tend to absorb oil in a fried product making the product greasy. In addition, pillowing and blistering can cause excessive breakage during packaging, shipping, and storing of a potato product. Still further, pillowing and blistering can cause dehydrated products to have a non-uniform moisture content and to rehydrate in a non-uniform manner. In addition, the blistered or pillowed areas tend to slough away during rehydration.
Various methods for reducing pillowing and blistering have been used in the preparation of conventional potato products. For example, pillowing and blistering in conventional potato chips has been controlled by adjusting the thickness of the potato slice, and the frying temperature. In general, thinner potato slices, produce less pillowing and blistering. However, some consumers prefer relatively thick potato chips, such that making the potato slices thinner is not always a satisfactory option. Lowering the frying temperature can also reduce pillowing and blistering, but can also increase cooking times and increase oil absorption.
Another method of controlling pillowing and blistering in conventional potato products is by increasing the moisture content of the potato slices, such as by soaking the slices in a salt solution. This slows the cooking process and inhibits blister formation. However, the taste and texture of the finished potato product can be adversely affected by the increased moisture content.
It is also known in the food preparation art that blistering and pillowing in some food products can be reduced by perforating the product with openings. The openings provide conduits for venting steam, thus preventing formation of the pillows and blisters. The process of perforating the product is referred to as "dockering", and the perforated product is referred to as being "dockered".
Typically, dockering has been employed with products that are made using some type of dough. For example, pizza can be made using dockered flour dough, as described in U.S. Pat. Nos. 4,170,659; 4,395,216; and 3,880,069. Fried corn chips can be made using dockered masa dough as described in U.S. Pat. No. 2,905,559. Fabricated potato chips, can be made using dockered potato dough, as described in U.S. Pat. Nos. 4,650,687; 4,931,303; 4,650,687; and 4,889,737. Dockering has also been employed in the dehydration of potatoes as described in U.S. Pat. Nos. 3,988,953 and 4,086,368.
The present invention adapts the dockering process to the preparation of potato products made with raw potato slices rather than dough. In addition, the dockering process can be controlled to produce openings having a conical shape, which facilitates moisture release from the potato products during cooking or dehydration, and moisture absorption during rehydration.